Oropharyngeal candidiasis caused by Candida albicans is prevalent in patients receiving cancer chemotherapy, AIDS patients, diabetics, or premature infants. Human salivary Histatin 5 (Hst 5) is a potent fungicidal protein that lacks toxicity to humans and has the ability to kill azole-resistant Candida strains. Our previous studies have demonstrated that Hst 5 exerts antifungal activity through a complex multi-mechanistic process. Toxic activity is initiated by binding to a C. albicans 70 kDa protein in the cell envelope that we have recently identified as Ssa1, a member of the HSP70 family involved in binding and transport of proteins across cell membranes. Following intracellular translocation, Hst 5 disrupts membrane ion gradients, perhaps through Trk1, resulting in disruption of volume regulation and G1 cell cycle arrest. However, many gaps remain in our understanding of this process. We do not know the critical requirements for Hst 5 binding with Ssa1 proteins. Delineation of the precise binding sites of Hst 5 with Ssa protein are crucial in design of antifungal toxic proteins targeted to the yeast cell surface receptors. In addition, we do not yet know the identity of yeast channels that participate in ion homeostasis disrupted by Hst 5, nor do we understand the global changes that occur ultimately leading to cell cycle arrest. To more fully understand these processes, we propose to 1) Identify the Hst 5 binding domain in Ssa1p in vitro 2) Assess the role of Ssa1/2p required for Hst 5 function in vivo, 3) Determine the role of the major yeast potassium channel, Trk1p, in Hst 5 killing and 4) Identify yeast gene transcripts, especially cell cycle regulatory genes, altered by Hst 5 using full genome C. albicans microarray analysis. An understanding of the mechanism of Hst 5-induced killing and the nature of its specificity for yeast cells over human cells will contribute to future development of improved antifungal drugs.